Recently, technology for storing holographic digital data is being advanced in various fields with the aid of the development of a semiconductor laser, a Charge Coupled Device (CCD), a Liquid Crystal Display (LCD) and the like. For example, a fingerprint recognition system for storing and reproducing fingerprints has been put to practical use. That is, the holographic digital data storage technology is adopted to the various fields in which the high storage capacity and very high data rate are required. In the Holographic Digital Data Storage (HDDS) system for storing large amounts of data, an interference pattern, generated by allowing a signal beam to interfere with a reference beam, is recorded on a holographic medium such as a photorefractive crystal, which is sensitive to the amplitude of the interference pattern. The HDDS system can display the three dimensional (3D) image of an object by using several hundred to several thousand pieces of hologram data, which are stored on the holographic medium in the form of a binary two dimensional array (referred to as a page).
FIG. 1 is a diagram showing the configuration of a conventional HDDS system. Referring to FIG. 1, the conventional HDDS system includes a light source 10, a beam expanding lens 12, a polarization beam splitter (PBS) 14, a spatial light modulator (SLM) 16, a condenser lens 18, a first and a second mirrors 20 and 24, a delay lens 22, a holographic medium 26, an objective lens 28, and a charge coupled device (CCD) 30.
A laser beam generated by the light source 10 passes through the beam expanding lens 12, and then is divided into a reference beam and a signal beam by the PBS 14. The signal beam is inputted to the SLM 16 and then modulated into a page of binary data (bright and dark) by the SLM 16. Thereafter, the modulated signal beam is provided to the holographic medium 26 through the condenser lens 18.
Meanwhile, the first mirror 20 reflects the reference beam, which is provided by the PBS 14, toward the second mirror 24 via the delay lens 22. Thereafter, the second mirror 24 adjustably reflects the reflected reference beam toward the holographic medium 26. Herein, the angle of the second mirror 24 is adjusted by an actuator (not shown).
The interference pattern generated by the interference phenomenon between the signal beam transmitted from the SLM 16 and the reference beam reflected by the second mirror 24 is recorded on the recording layer of the holographic medium 26. In this case, the interference pattern is recorded on the recording layer of the holographic medium 26 through the light-induced generation of a mobile charge in the holographic medium 26 in response to the amplitude of the interference pattern.
Thereafter, the conventional HDDS system reproduces data recorded on the holographic medium 26 in the following manner. In order to reproduce the data recorded on the holographic medium 26, only the reference beam should be irradiated onto the holographic medium 26 while the signal beam is blocked. For this purpose, the signal beam provided from the PBS 14 may be blocked by closing a shutter (not shown) located between the PBS 14 and the SLM 16, and the reference beam provided from the PBS 14 is allowed to be incident on the first mirror 20 by opening another shutter (not shown) located between the PBS 14 and the first mirror 20. The reference beam reflected through the first and the second mirrors 20 and 24 in that order is incident on the holographic medium 26 at an incidence angle identical to that used during the recording operation. Then, the interference pattern recorded on the recording layer of the holographic medium 26 diffracts the reference beam, so that the original data, i.e., the page of binary data (check board-shaped pattern), can be reproduced. The reproduced page of binary data is provided to the CCD 30 via the objective lens 28, and then the CCD 30 takes the image of the reproduced page to convert the image into electrical data.
According to the conventional HDDS system, the angle of the second mirror 24 should be adjusted to allow the reference beam to be incident on the holographic medium 26 during playback. In the conventional HDDS system, the angle of the second mirror 24 may be adjusted to be identical to that during the recording operation. However, a unit for measuring the error in the angle of the second mirror 24 and performing the servo control for compensating the error is not included in the conventional HDDS system. Therefore, the conventional HDDS system is problematic in that it is difficult to reproduce the data accurately.